Why colors of a Blender exported .obj mesh differ in Meshlab and Sculpteo?
Well, I think I found the answer by myself:

It's well known that the colors we see on a Blender rendered image are not the colors we have given to the material. Generally speaking this is due to World's impact. Blender exports in .obj format without taking in view this factor. It does not export the colors you have set in advance, but the colors they change to, when the impact of environment is not there. So even if you open your exported .obj file with Blender you will see your mesh with changed colors (darkened). So in order your .obj file to open the mesh with desired colors lighten them in Blender before exporting.

:)